The present invention relates to a method of manufacturing a curved tubular bulb and, more particularly, to a method of manufacturing a curved tubular bulb having a compressed cross section.
As a fluorescent lamp device having compatibility with an incandescent lamp, a fluorescent lamp device having a convoluted discharge path is used. An example of a fluorescent lamp device of this type is disclosed in U.S. patent application Ser. No. 912,126 filed Sept. 26, 1986. This fluorescent lamp device is a convoluted fluorescent lamp comprising a plurality of U-shaped tubular bulbs The U-shaped tubular bulb has a compressed cross section The bulb comprises a tube curved along a U-shaped curve existing in a plane and having an oblong cross section compressed in the direction normal to the plane. Since the bulb has such a cross-sectional shape, the relative surface area to the cross-sectional area of the bulb is increased, thereby improving the heat dissipation property of the fluorescent lamp. As a result, thermal degradation of a phosphor coating applied inside the fluorescent lamp is suppressed, thereby allowing an increase in the lamp input and hence light-emitting intensity. Further, such cross-sectional shape of the bulb makes good use of the light emitted therefrom.
According to the method described in this application, a U-shaped bulb is entirely molded from a glass tube by means of molding dies A. continuous wall is formed between straight portions of the U-shaped bulb. Since the continuous wall exists between straight portions of the bulb, it is difficult to uniformly heat the bulb, particularly end portions of the bulb. For this reason, electrode mounts and others cannot be reliably connected to the bulb, or end portions of the bulb. Therefore, the step of partially removing the continuous wall is essential.
Japanese Patent Disclosure (Kokai) No. 57-37449 discloses a spiral bulb for a mercury-vapor lamp having a compressed cross section This bulb comprises a tube curved into a spiral in shape and having an oblong cross section compressed in the curved direction. That is, the bulb is curved in the direction of a minor diameter of the oblong section. In other words, this bulb for a mercury-vapor lamp is different in compressed direction than that of the U-shaped bulb for a fluorescent lamp.
This bulb for a mercury-vapor lamp can be manufactured by a conventional method which is known to those skilled in the art. That is to say, a heated straight glass tube is bent about a heated jig to form a spiral glass tube. With the tube bending, the tube is deformed to have a compressed cross-sectional shape. If the above mentioned U-shaped bulb for a fluorescent lamp is manufactured by such a method, a difference of curvatures between outermost and innermost surface of the curved glass tube becomes very large. For this reason, the wall thickness of the manufactured bulb locally varies, resulting in a very high percentage of defective products.